1. Technical Field
Aspects of this disclosure relate to the use of radar in monitoring physiological conditions of mammals.
2. Related Art
Animals, like humans, can suffer from injury or illness, negatively impacting their health. Timely detection of changes in health through either regular or event driven episodic monitoring of physiological processes can enable veterinary intervention, potentially reducing the effects of an adverse condition, improving the quality of life, and prolonging life. In particular, cardiac and respiratory monitoring provides useful information on the health of an animal and these types of information are commonly used to diagnose, treat, and manage the animal.
Animals can represent a large financial and often, emotional investment. Health monitoring can help optimize veterinary care to protect that investment and provide peace of mind to the owner. Monitoring is applicable to a wide range of animals, including feed stock, breeding stock, exotic/endangered species, animal athletes, performing animals, and domestic pets. Monitoring can be accomplished whether the animal is in the wild, in captivity (e.g. a zoo or animal park), in a pasture or free-range, in a barn or stable, at home or in the yard, in a pen or a crate.
Animal health monitoring is challenging. Many familiar sensor technologies—e.g. ECG, pulse oxygen, ultrasound, and temperature, require direct skin contact, making them impractical for use with animals having fur or feathers. They may also require that the sensor be positioned on a specific location on the body which again may be impractical. For example, assuming prior removal of the fur, pulse ox sensors typically need to be placed on thin anatomical structures such as ears, making them prone to loss through scratching, rubbing, or shaking. Similarly, ECG sensors are usually placed on the torso, in proximity to the heart and the Einthoven triangle, making them prone to loss through scratching, rubbing, or shaking. Finally, there is no sensor currently available that is capable of providing a direct, unobtrusive measurement of respiration—a needed metric in understanding and managing animal health.
The monitoring of respiration is currently under-appreciated in veterinary care and there are only a handful of researchers in the country studying/teaching animal pulmonology. This lack of integration into veterinary medicine is in contrast to the body of published information dating back several decades concerning the role of respiratory symptoms in the diagnosis and treatment of animal cardiac and respiratory disease. One of the obstacles in integrating respiratory monitoring into veterinary practice is the lack of appropriate non-invasive sensors. Most veterinarians are forced to rely on manual observations—watching the animal, to obtain respiratory data. These observations are of limited use and complicated by the visit to the vet as this usually leads to animal anxiety and elevated cardiopulmonary functions that are not representative of the animal's true underlying health. Respiratory monitoring is not viewed as an important parameter because of the difficulty in obtaining accurate data.
Respiratory monitoring in the animal's nature environment—e.g. at home for a pet or in a pasture for a horse or cow, would be a benefit to veterinary medicine as the data would be more representative of the animal's actual state of health. This data could be used to help treat animals with known medical problems as well as identify animals that may be developing medical problems. There are a number of medical problems that exhibit respiratory symptoms, including heart disease, heart murmur, pulmonary edema, pulmonary fibrosis, sleep apnea, COPD, asthma, larynx paralysis, kennel cough (bordetella), and others. Specific to domestic pets, respiratory monitoring would be important with brachycephalic dogs—breeds with short muzzles such as bulldogs, cavaliers, pugs, Boston terriers, Boxers, Pekingese, shih tzu, etc. These breeds have a high incidence of respiratory problems and are inefficient “panters”, leading to inflamed respiratory tracts and laryngeal problems as well as making them much more susceptible to heat stroke. Timely identification of respiratory distress would enable earlier and less complicated/expensive intervention and reduce the risk to the animal.
As discussed above, many medical monitoring technologies are impractical or unusable with animals. Doppler radar approaches, whether CW or pulsed, have been investigated as a technique for collecting cardiopulmonary data. They have generally relied on off-body or non-contact monitoring where the Doppler radar sensor is separated from the subject by an air gap and thus, does not make direct contact with the patient. Due to the large difference between the relative dielectric properties of the primary propagation medium (air, where εr=1) and living tissue (εr≈50), most of the RF energy is reflected at the skin surface with little energy propagating into the interior of the body to interrogate the internal organs. Any energy that does propagate into the torso and is subsequently reflected by the internal organs is greatly reduced by internal tissue absorption as well as a second transition across the skin-air boundary, resulting in little energy from the anatomical target making it back to the receiver. Low returns equate to marginal data.
A common technique for isolating a specific physiological process involves combining Doppler with auto-correlation. Auto-correlation samples the time-domain waveform and correlates the Nth pulse with a period of time after the Nth pulse where the period is centered on the anticipated rate of the specific physiological process under review based on the Doppler results. High correlation coefficients equate to greater confidence that the system has locked onto the specific physiological process. An externally defined threshold is often used to determine adequate correlations and thus, sufficient target acquisition.
Because of the strong surface component associated with respiration (typically 1 cm chest wall displacement in an average adult male), off-body techniques can collect reasonable pulmonary data but those physiological processes that do not have strong surface components, such as cardiac activity, are difficult to detect and measure with
Doppler. Another limitation of Doppler is its general inability to distinguish motion associated with more than one physiological process when those processes operate at similar rates. For example, in subjects experiencing bradycardia, the cardiac rate will approach and sometime drop below the respiratory rate, making it difficult for Doppler to distinguish the two processes from each other.